In recent years, with the increased speed of wireless communications, the bandwidth and dynamic range of transmission signals have increased. Under these conditions, to reduce the degradation of the quality of signals to the minimum, power amplifiers having high linearity are needed. Furthermore, at the same time, from the viewpoint of reducing device size and the operation costs and from the viewpoint of environmental issues, there is also a growing need for power amplifiers that operate with high power conversion efficiency.
In commonly used power amplifiers, the linearity and the power conversion efficiency have a conflicting relationship with each other. For example, it is possible to reduce the amount of out-of-band distortion by operating a power amplifier in a linear region in which the power amplifier is backed off from its saturation level. However, in such a case, the power conversion efficiency is considerably low, thus increasing the power consumption of the power amplifiers. Accordingly, to have both satisfactory linearity and the power conversion efficiency, linearity should be maintained by operating the power amplifier in a nonlinear region in which the power conversion efficiency is high and also using distortion compensation that removes nonlinear distortion that is generated when the power amplifiers is operated in this nonlinear region. A predistortion method (hereinafter, referred to as a “PD method”), which is a method of the distortion compensation, is a technique for enhancing the linearity of output of a power amplifier by multiplying the inverse characteristics of the nonlinear distortion of the power amplifier by a transmission signal. The multiplication of the PD method mentioned here is done in complex region. An example of such PD methods includes a conventional technique for suppressing spurious emissions in accordance with the number of carriers, which is equal to the bandwidth of the transmission signal. Furthermore, there is a conventional technique for performing distortion compensation using look-up tables associated with multiple frequency bands.
In power amplifiers that can operate with high efficiency, it is known that a memory effect phenomenon occurs. A memory effect phenomenon is a phenomenon in which, in a power amplifier at a certain time, an output with respect to the input is affected by the past input data. Accordingly, when using distortion compensation, on a power amplifier that operates with high power efficiency, using a PD method that does not support the compensation of the memory effect in which a distortion compensation coefficient is determined only by using the signal amplitude obtained at the current time, it is difficult to obtain the desired distortion suppression effect. Accordingly, in order to obtain the desired distortion suppression effect by using a power amplifier that operates with high efficiency, distortion compensation using the PD method that supports the compensation of the memory effect (hereinafter, referred to as a “memory PD”) has been proposed in which a predistortion signal is generated by using, in addition to the current time, the past data. There is one type of memory PDs that has a transversal filter structure by using both a transmission signals and distortion compensation coefficients to generate a predistortion signal. In the description below, a predistortion signal generated by using the transversal filter is referred to as a transversal-filter-type predistortion signal.
In the following, an example of the operation of a conventional power amplification device that uses transversal-filter-type predistortion signals will be described. In this example, it is assumed that a predistortion signal generating unit is provided in both forward and feedback paths. The power amplification device obtains N distortion compensation coefficients by using a transmission signal that is delayed by a delay element by a maximum of N clocks. Then, the obtained distortion compensation coefficients are input to a predistortion signal processing unit in a forward path having a transversal filter structure. Then, the distortion compensation coefficients are multiplied by transmission signals having the same delay time. A signal obtained by adding each of the multiplication results is output as a predistortion signal. The predistortion signal generating unit in the forward path generates, using the transmission signal and the distortion compensation coefficients provided by the coefficient generating unit, a predistortion signal having the inverse characteristics of the nonlinear distortion of the power amplifier. Then, the predistortion signal in the forward path is subjected to digital-to-analog (DA) conversion; is up-converted to a carrier frequency; and then is input to the power amplifier. A part of the output from the power amplifier, in which nonlinear distortion is removed due to the predistortion, is looped back by a directional coupler; is subjected to down-conversion, analog to digital (AD) conversion, and demodulation; and becomes a digital feedback signal. The predistortion signal generating unit in the feedback path has the same configuration as that of the predistortion signal generating unit in the forward path. The predistortion signal in the feedback path performs predistortion on the feedback signal. Then, the power amplification device obtains update information on the distortion compensation coefficients to minimize the power of an error signal that is the difference between the predistortion signals of the forward path and the feedback path.
Furthermore, for amplifiers that use transversal-filter-type predistortion signals, there is also a conventional technique for obtaining a distortion compensation coefficient using a power series. Furthermore, there is also a conventional technique for generating a predistortion signal by obtaining in-band and out-of-band signals subjected to predistortion.    Patent Document 1: Japanese National Publication of International Patent Application No. 2006-505160    Patent Document 2: Japanese Laid-open Patent Publication No. 2007-13947    Patent Document 3: Japanese Laid-open Patent Publication No. 2007-20157    Patent Document 4: Japanese Laid-open Patent Publication No. 2006-246398    Patent Document 5: U.S. Pat. No. 6,141,390    Patent Document 6: U.S. Pat. No. 6,356,146
In a conventional distortion compensation method using a transversal-filter-type predistortion signal, delay time τ due to a tap that gives a delay to the input signal is determined on the basis of the maximum bandwidth of the transmission signal and the tap interval is fixed regardless of a change in transmission signal properties. This applies to both the conventional technique that uses a power series and the conventional technique that performs predistortion on in-band and out-of-band signals. Accordingly, if the correlation between adjacent data sample points of an input signal is high, i.e., if the signal bandwidth is narrow due to a change in carrier configuration or if the power of a control channel is significantly high with respect to the power of a data channel, there may be a case in which distortion compensation coefficients may not converge to their optimum values. The reason for this is that, if the correlation between the adjacent data sample points of an input signal is high, the values of the adjacent data sample points of signals that are output from taps becomes very similar. Accordingly, when updating a distortion compensation coefficient, the effect of the adjacent data sample points and other distortion compensation coefficients becomes strong; therefore, in a coefficient update algorithm, such as a least mean square (LMS) algorithm, that is based on a steepest-descent method, the amount of the update to the coefficients is not correctly obtained. As described above, because the distortion compensation coefficients are not converged to their optimum values, there is a problem in that distortion suppression performance is degraded.